A dual mode transmitter can operate using two different systems. For example, an AM/FM dual mode transmitter can transmit both amplitude modulated and frequency modulated signals. For radiotelephones, a dual band transmitter can operate using two different cellular telephone systems. For example, a dual band GSM/DCS radiotelephone can use the Global System for Mobile Communications (GSM), which operates at 900 MHz, and the Digital Communications System (DCS), which is similar to GSM except that it operates at 1800 MHz.
In any radiotelephone, the power amplifier at the final stage of the transmitter should be matched to the impedance of the antenna. Additionally, harmonics of the transmitted frequency band should be suppressed to reduce interference with other communication systems operating at the harmonic frequencies. With a GSM/DCS dual band transmitter, it is difficult to suppress the first (1800) MHz harmonic during 900 MHz GSM transmissions and yet pass the 1800 MHz signal during DCS transmissions. Also the output impedance of a radiotelephone power amplifier should be matched to the antenna so that the impedance at the output of the amplifier is at the optimum impedance for power efficient amplification.
Thus, there is a need for a dual band power amplifier that can suppress harmonic frequencies during a first mode of transmission and also properly pass signals during a second mode of transmission, even when the signals of the second transmission are at or near a harmonic frequency of the first mode of transmission. There is also a need for a dual mode power amplifier with a limited number of parts and a low current drain.